The invention relates to an instrument for measuring the thickness of coatings on a substrate on the basis of the electromagnetic interaction between a measuring probe resting on the coating and the substrate as a function of the spacing between probe and substrate.
Such conventional instruments are used in tow designs: namely, one for measuring the thickness of non-magnetic coatings, such as galvanized layers, paints, etc. on a magnetic substrate, in particular, steel and nickel, and another for measuring the thickness of insulating coatings on electrically conducting substrates.
When measuring the thickness of non-magnetic coatings on a magnetic substrate, the magnetic flux in the measuring probe varies as a function of the spacing between it and the magnetic substrate, hence as a function of the layer thickness. This variation in magnetic flux is detected, for instance, from the frequency of a magnetic switch circuit and converted into a signal which is displayed.
When measuring the thickness of non-conducting coatings on a conducting substrate, eddy currents are generated in the conducting substrate by a high-frequency AC powered measuring probe and are reflected into said probe. This feedback depends on the thickness of the layer, so that it can be converted into a corresponding electrical signal which again is displayed.
However, these known instruments suffer from various drawbacks. Thus, the dependency of the measured signal on layer thickness is approximately, but not precisely, a hyperbola. Accordingly, there will be a very pronounced change in the measured signal for extremely thin layers whereas, for thicker ones, the variation in the measured signal will only be minute. To compensate this unfavorable functional behavior in display, the initial measuring range frequently is expanded by bias-magnetization, insertion of shims of various materials or similar steps. However, the instrument complexity is substantially increased thereby. Furthermore, the measured signal cannot be digitized in simple manner because, as already mentioned, the measured function deviates from the hyperbolic shape. Therefore, special steps must be taken to correlate the output signal with a corresponding digital value.
Because of the high cost required to that end, digitally displaying instruments so far have not been marketed.
The known instruments suffer from a further problem, namely that they are extremely sensitive circuits which measure inaccurately, expecially if there are temperature fluctuations. These factors to-date could be remedied only at great cost. Finally, these instruments are also very sensitive to changes in the battery supply potential, requiring additional steps and, for instance, a display to indicate the end of battery life.